The invention relates to a water flow heater as defined in the preamble of claim 1.
Such a water flow heater is known from the German patent application DE 2 837 934. This water flow heater is designed for heating water which flows through the heating range of the water flow heater. A first heater element has a higher power than a second heater element and is situated upstream of said second heater element. Downstream of the second heater element there is a third heater element. The second heater element in its turn has a greater power than the third heater element. Temperature sensors are placed downstream of the second heater element and of the third heater element for scanning the temperature of the water heated by the heater elements.
The first heater element is switched on in response to the flow through the channel of a certain minimum quantity of water and is not controlled. The second heater element is controlled with a frequency of 10 minxe2x88x921 in dependence on the water temperature sensed by the temperature sensor directly downstream of said second heater element in the channel. The third heater element is controlled with a frequency of 100 minxe2x88x921 in dependence on the water temperature sensed by the temperature sensor directly downstream of said third heater element in the channel.
Since the heater elements are controlled with a lower frequency in proportion as the power is greater, and since the heater element with the highest power is controlled independently of temperature, a limitation of the interference with the public mains is achieved. Such interferences are unpleasant in particular because they lead to variations in the light output of lamps, which fluctuations are visible to the human eye at very small voltage fluctuations already and are perceived as unpleasant.
The German patent application 34 26 046 and European patent 0 630 463 also disclose water flow heaters for heating water with several heating zones in succession, in which exclusively a portion of the heating zones is controlled in dependence on the temperature of water supplied by the water flow heater, while the other portion of the heating zones is activated exclusively in reaction to the detection of a certain minimum flow of water.
It is a disadvantage of this known water flow heater that the water temperature often does not come up to a user""s expectations in the case of a low flowrate and becomes badly predictable. The water temperature becomes either too high, because too much power is supplied for the water flowrate, or too low, because the water flowrate lies below the lower limit below which the first heating zone is switched off. This problem becomes stronger in proportion as the first heating zone has a comparatively greater power, and the relative control range of the water flow heater is smaller. A high power of the non-controlled heating zone in proportion to the total power is advantageous, however, because in that case only a comparatively small power is to be controlled. It is suggested in the German patent application 34 26 046 to switch certain heating zones on and off in dependence on the water flowrate, but this complicates the construction and increases the number of switching operations, as does the temperature-dependent control, with the result that voltage fluctuations occur in the public mains.
The invention has for its object to provide a simple solution which renders it possible to implement such a control at low cost in, for example, small domestic appliances such as coffee-makers. According to the invention, this object is achieved in that a water flow heater is constructed in accordance with claim 1.
The use of a pump for generating the water flow through the channel and the activation of the heater elements in conjunction with the activation of the pump achieve in a simple manner that, if there is a water flow through the channel, the water flowrate will at least have a certain value and will always lie within a predetermined, comparatively narrow bandwidth, at least after a starting phase. This renders it possible to use a comparatively large, non-controlled heater element without the undesirable consequences of an occasionally unexpectedly high or low water temperature arising.
The necessity of complicated and vulnerable detectors for determining the water flowrate and of separate heater elements which are controlled in dependence on the water flowrate is thus also avoided.
Particularly advantageous embodiments of the invention are described in the dependent claims.